The present invention relates generally to a flow sensing assembly and method for detecting the flow of fluid through a flow passage and, more particularly, to a flow sensing assembly and method for detecting the presence of fluid flow in low flow applications.
This invention has particular (albeit not exclusive) application to the lubrication of bearings. There are a variety of systems on the market which are used for delivering lubricant to bearings. In many cases it is desirable to monitor whether lubrication is accomplished in a timely manner, especially with respect to critical bearings which might otherwise fail. Monitoring can be carried out by manual examination of the bearing or by a system which automatically detects flow of lubricant to the bearing. However, where the quantities of lubricant delivered to the bearing are small, reliable detection is difficult to achieve. This is due at least in part to the unavailability of an effective yet inexpensive sensor which is capable of detecting the flow of even small quantities of material through a flow line.
Reference may be made to U.S. Pat. Nos. 2,943,486 and 4,143,255 disclosing various devices in the field of this invention. U.S. Pat. No. 2,943,486 describes a flow sensor which is designed to extend into a flow passage and obstruct the path of fluid flow so that the flow rate of the fluid may be determined. While able to detect the presence of fluid flow under high flow conditions, such sensors typically do not extend across the entire flow passage and thus do not effectively detect the presence of fluid flow in low flow applications. Consequently, the use of these sensors in a lubrication line would likely result in excessive lubrication, increased expense and reduced performance.
U.S. Pat. No. 4,143,255 discloses a flow sensor having a spring-biased paddle which extends into and across the flow path during static or "no-flow" conditions. The flow of fluid causes the paddle to align itself parallel with the fluid flow during "flow" conditions, and an associated switch or magnet determines whether the paddle is in the "no-flow" or "flow" position. However, these sensors do not effectively detect the presence of fluid flow in low flow applications. As an example, low flow may fail to pivot the paddle into the "flow" position because of the force of the spring in addition to the static fluid pressure on the downstream side of the paddle. Moreover, once the paddle is in the "flow" position, the upstream static fluid pressure may prevent the paddle from returning to its "no-flow" position. This is especially true in applications involving highly viscous materials such as a lubricant. Sensors which use a magnet to signal movement between the no-flow and flow positions suffer from the further drawback that a nonmagnetic material must be used so that the sensor will not interfere with the magnetic field.